A collapsed lava tube along the Lava Flow Trail. Close by is a cave created by a similar collapse of the lava tube ceiling. I was hoping to explore it. However, a sign at the opening advised having three in your party, several light sources, first aid kits, and a reminder that cellphones don't work underground. In other words, if you get hurt, you have to save yourselves.
Also, the entrance was a deep hole with a tiny low ceilinged ledge on one side that you had to crawl along. Since I was a party of one, I was glad to stay on the surface.
Snow Canyon State Park
Saint George, Utah